Process for controlled removal of carbon under vacuum from highly alloyed steels

ABSTRACT

In a process for removing carbon from a highly-alloyed steel, such as a chrome steel, the steel is treated by blowing oxygen into or on to the molten steel in a vacuum vessel to remove carbon from the melt. The temperature of the gases extracted from the vessel is monitored to detect an abrupt drop in the gas temperature, at which point the oxygen treatment is arrested and the temperature of the melt is immediately measured. The composition of the melt is then corrected on the basis of the known equilibrium concentration of carbon and chrome, or other constituent, which occurs at that measured melt temperature. The composition of the melt may be corrected either by adding further alloying substances thereto, or by further oxygen treatment, as the situation requires.

[4 1 Sept. 24, 1974 PROCESS FOR CONTROLLED REMOVAL OF CARBON UNDER VACUUM FROM HIGHLY ALLOYED STEELS [75] Inventor: Horst Kutscher,

Dortmund-Berghofen, Germany [73] Assignee: Vacmetal Gesellschaft fur Vakuum-Metallurgie m.b.H., Dortmund, Germany 22 Filed: Mar. 17, 1972 21 Appl. No.: 235,506

[30] Foreign Application Priority Data Mar. 25, 1971 Germany 2114600 [52] U.S. Cl. 75/49, 75/60 [51] Int. Cl. C2lc 5/30, C21c 7/10 [58] Field of Search 75/60, 49.

[56] References Cited UNITED STATES PATENTS 3,003,865 10/1961 Bridges 75/60 3,450,867 6/1969 Blum 75/60 3,528,800 9/1970 3,640,119 2/1972 3,645,718 2/1972 Murphy 75/60 3,666,439 5/1972 Ramachandranm. 75/60 3,669,645 6/1972 Oishi 75/60 Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or FirmToren & McGeady [5 7] ABSTRACT In a process for removing carbon from a highlyalloyed steel, such as a chrome steel, the steel is treated by blowing oxygen into or on to the molten steel in a vacuum vessel to remove carbon from the melt. The temperature of the gases extracted from the vessel is monitored to detect an abrupt drop in the gas temperature, at which point the oxygen treatment is arrested and the temperature of the melt is immediately measured. The composition of the melt is then corrected on the basis of the known equilibrium concentration of carbon and chrome, or other constituent, which occurs at that measured melt temperature. The composition of the melt may be corrected either by adding further alloying substances thereto, or by further oxygen treatment, as the situation requires.

7 Claims, 1 Drawing Figure PATENIEBSEPEMHH 0 7 $837; 841

1600 who 1860 "c TEMPERATURE OF THE MELT PROCESS FOR CONTROLLED REMOVAL OF CARBON UNDER VACUUM FROM HIGHLY ALLOYED STEELS This invention relates to a process for removing carbon from highly-alloyed steels by blowing gaseous oxygen on to or into the molten steel under vacuum.

Processes are known in which carbon is removed under vacuum from highly-alloyed steels by blowing oxygen on to or into the molten steel. A difficulty arises, however, in controlling the known processes so as to obtain the desired final carbon concentration in the steel. Hitherto it has been necessary to work by estimations. However, the operating conditions fluctuate widely due to the influences of a variety of factors, for example the concentration of oxygen in the melt, and consequently the desired carbon concentration'in the final steel can easily be overshot or undershot. When this happens, it is necessary to take several samples of the steel under vacuum and await the results of analysis, whereupon the carbon concentration is adjusted to the desired value by a further blow of oxygen or by adding carbon to the melt.

It is an object of the present invention to provide a process in which the desired final carbon concentration is obtained directly and reliably without any need to take samples of the steel during the process.

In the process according to the invention, a high carbon alloy steel which has been refined in the conventional manner is introduced into a vacuum vessel and carbon is removed until the extracted gases show an abrupt temperature drop, whereupon th melt temperature is measured and the composition of the melt is corrected on the basis of the known equilibrium concentrations of carbon and chromium or other constituent at the measured melt temperature.

A process in accordance with the invention will now be described in detail with reference to the accompanying diagram which shows the variation of the percentage of carbon in the steel against temperature of the melt for different chrome contents.

Starting out from a chrome alloy steel, this is melted in a furnace in the known way, the concentrations of chromium, nickel, phosphorous and sulphur being adjusted approximately to the desired values. During the refining of the steel in the furnace, the carbon concentration in the melt is considerably higher than the desired final concentration, to prevent excessive loss of chromium. In the furnace the carbon concentration can, for example, be between 0.2 and 1 percent. When the treatment in the furnace has beem completed, the melt is tapped off with little or no slag and introduced, for example, into a ladle in a degassing apparatus in which the melt is degassed, if necessary with the help of a current of argon. During the degassing process the absolute pressure over the melt decreases down to ap proximately Torr. When the pressure has been brought down to this value, oxygen is introduced into the vacuum vessel by means of a lance, the oxygen being blown either on to the surface of the melt or into the body of melt. The resulting reaction raises the temperature of the melt by an amount which depends on the rate of feed of oxygen.

The reaction:

is exothermal, the heat released raising the temperature of the melt and raising the temperature of the gas mixture extracted from the vacuum vessel.

The rise in the temperature of the extracted gas is utilised for monitoring the process of carbon removal. The vacuum pumps can, for example, be dimensioned so as to ensure that, in spite of the CO produced, the pressure in the vacuum vessel remains between 10 Torr and 30 Torr. It has been found by making tests that, in large scale production, if the pressure is held between these two values the carbon concentration in the melt is brought down with economically tolerable rapidity to a value at which the CO equilibrium corresponds to a pressure of approximately 50 Torr. At this pressure, the carbon concentration in the melt depends on the oxygen concentration in the melt and this depends on the melt temperature and on the chromium concentration in the melt.

At a given CO partial pressure, the highest oxygen concentration which is possible without losing chromium into the slag increases with melt temperature. Consequently the final carbon concentration is lower as the melt temperature increases. However, the desired equilibrium final carbon concentration is reached only after blowing with oxygen and evacuating the gases produced for long enough to complete the CO reaction. When equilibrium is reached, the CO reaction ceases. Further blowing then results in loss of chromium into the slag. The attainment of equilibrium is indicated by an abrupt temperature fall in the extracted gases, due to the fact that the exothermal reaction between the carbon and the oxygen, with evolution of CO, has ceased.

The carbon concentration in the melt is monitored by measuring the melt temperature at the instant of the temperature fall in the evacuated gases. If the carbon concentration is too low, that is to say if it is below the desired final value, carbon can be added without delay to the melt under vacuum. On the other hand, if the carbon concentration is too high there is added to the melt under vacuum an element which has a high affinity for oxygen and reacts with oxygen strongly exothermally, for example silicon. The quantity of silicon added to the melt is chosen so that, in its reaction with the oxygen fed into the melt, sufficient heat is produced to raise the temperature of the melt to the temperature which corresponds to the desired carbon concentration. After the element has been added to the melt, oxygen is blown briefly, raising the melt temperature and lowering the carbon concentration down to the desired value. Once more the attainment of equilibrium is indicated by an abrupt drop in the temperature of the extracted gas.

The process described above can, of course, if desired be conducted in a different way. The required initial melt temperature at the beginning of the vacuum treatment can be calculated from the final melt temperature corresponding to the desired carbon concentration, the heat which can be expected to be lost during the degassing without oxygen feed, and the heat which will be released by the CO reaction. When the calculation has been completed, the initial melt temperature is adjusted. If the melt in the furnace is too hot, its temperature can be adjusted downwards during the tapping off procedure. If the melt in the furnace is too cool, its temperature can be adjusted upwards, either before or during the vacuum treatment by adding the quantity of silicon necessary to give the correct final melt temperature, at the end of the vacuum treatment, corresponding to the desired final carbon concentration. The initial melt temperature, at the beginning of the vacuum treatment, can be calculated in the known way from the following equation:

where:

T is the calculated tapping temperature.

T is the final temperature corresponding to the de' sired final carbon concentration, as represented in the diagram.

T, is the temperature drop occurring during tapping off from the furnace.

T is the loss of temperature in the melt in the vacuum pan between the end of the tapping procedure and the end of the blow period. The blow period can be assumed with sufficient accuracy to be between and minutes. The quantity T is the sum of several operational quantities which can easily be measured.

T is the total heat of reaction released by the reaction between oxygen and carbon and by the reactions between oxygen and other elements. The quantity T can be calculated from the following equation:

T 300 Si] 140 C] 350 Al] 130 Cr] 90 Mn] 2.

It will be observed that if the tapping temperature is too low, it is not necessary to prolong furnace time in order to raise the tapping temperature. The final melt temperature can be corrected by adding silicon or aluminum to the melt under vacuum before blowing with oxygen. This increases the quantity T and counteracts the excessively low tapping temperature T The following examples will serve for clarifying the nature of the process according to the invention. Example 1 A 50 tonne melt of the following composition was tapped from a furnace:

0.5 percent Carbon 0.35 percent silicon 0.5 percent manganese 18 percent chromium 10 percent nickel 0.015 percent sulphur 0.03 percent phosphorous.

A melt was tapped at a temperature of 1,630C. The ladle was introduced into a vacuum vessel and a vacuum applied.

The melt contained a certain concentration of oxygen and consequently a slight CO reaction took place, resulting in the carbon concentration decreasing by approximately 0.02 percent. As soon as the pressure had decreased to 10 Torr, an oxygen lance was introduced into the vacuum vessel over the surface of the melt. Oxygen was fed at a rate of 1,800 kg/h. A strong CO reaction took place, the effluent gas temperature rising from an initial 300C up to approximately 500C. At the end of approximately 12 minutes, the effluent gas temperature abruptly dropped. The oxygen feed was immediately cut off. The temperature of the melt was found to be 1,770C. From the diagram this corresponds to a carbon concentration in the melt of 0.01 percent. The composition of the melt was corrected by adding:

500 kg ferromanganese (80 percent Mn. 1 percent C) 150 kg ferrochrome percent Cr, 1.5 percent C) 480 kg ferrosilicon percent Si).

This gave the melt the final composition:

0.025 percent carbon 0.7 percent silicon 1.3 percent manganese 18 percent chromium 10 percent nickel 0.03 percent sulphur 0.03 percent phosphorous.

The carbon concentration agrees very well with the specified concentration of 0.02 to 0.03 percent C. Example 2 A 50 tonne melt tapped from the furnace had the composition:

0.3 percent carbon 0.5 percent silicon 1 percent manganese 17 percent chromium 10 percent nickel 0.01 sulphur 0.025 percent phosphorous.

The tapping temperature was 1,600C. The pressure in the vacuum vessel was brought down to 10 Torr, whereupon oxygen was fed at the rate of l .000 kg/ h. At the end of 10 minutes the effluent gas temperature dropped, the melt now showing a temperature of 1,670C. From the diagram this corresponds to a carbon concentration of 0.022 percent. The specification called for a carbon concentration of at most 0.03 percent, with at least 17.5 percent chromium. It was therefore necessary to add at least 1 percent of chromium. Ferrochrome was added containing 1.5 percent carbon, involving an increase of the order of0.021 percent in the carbon concentration. There were therefore added to the melt 750kg of ferrochrome (70 percent Cr) and 130 kg of ferrosilicon (75 percent Si). This addition resulted in the temperature of the melt decreasing to 1,630C, the carbon concentration increasing to 0.05 percent. A further brief blow of oxygen was therefore given, the oxygen flowing at the rate of kg/h. The pressure increased to approximately 25 Torr and after only 2 minutes the temperature decreased considerably. Finally the melt showed a temperature of 1,650C, corresponding to 0.025 percent carbon. Finally there were added to the melt under vacuum 500kg of ferrosilicon (75 percent Si) with mixing of the melt. The final melt analysis showed the composition:

0.025 percent carbon 0.7 percent silicon 0.9 percent manganese 17.8 chromium 10 percent nickel 0.01 percent sulphur 0.025 percent phosphorous.

This agreed with the specification.

The process therefore makes it possible to control the vacuum treatment in such a way that the desired low carbon concentration is obtained, without timeconsuming analyses and without serious chromium losses. The process is therefore particularly suitable for application in high output technology.

The process is also applicable to other types of highly-alloyed steels, besides the chrome steel described above.

We claim:

1. A process of removing excess carbon from alloy steel containing an excess amount of carbon, which comprises:

a. introducing oxygen into a melt of said alloy steel while the melt is under vacuum conditions, whereby gases are extracted from the melt;

b. monitoring the temperature of the extracted gases;

of the melt at the measured melt temperature of (d).

3. A process as claimed in claim 2, in which the composition of the melt is corrected by adding alloying substances thereto.

4. A process as claimed in claim 2, in which the composition of the melt is corrected by subjecting the melt to further oxygen treatment.

5. A process as claimed in claim 4, in which before the melt is subjected to said further oxygen treatment an element which reacts exothermally with oxygen is introduced into the melt.

6. A process as claimed in claim 1, in which said oxygen is introduced by blowing oxygen into or on to the melt.

7. A process as claimed in claim 2, in which the steel is a chrome steel, and the composition of the melt is corrected on the basis of the known equilibrium concentrations of carbon and chromium at the measured melt temperature. 

2. A process as claimed in claim 1, further comprising correcting the composition of the melt on the basis of the known equilibrium concentration of constituents of the melt at the measured melt temperature of (d).
 3. A process as claimed in claim 2, in which the composition of the melt is corrected by adding alloying substances thereto.
 4. A process as claimed in claim 2, in which the composition of the melt is corrected by subjecting the melt to further oxygen treatment.
 5. A process as claimed in claim 4, in which before the melt is subjected to said further oxygen treatment an element which reacts exothermally with oxygen is introduced into the melt.
 6. A process as claimed in claim 1, in which said oxygen is introduced by blowing oxygen into or on to the melt.
 7. A process as claimed in claim 2, In which the steel is a chrome steel, and the composition of the melt is corrected on the basis of the known equilibrium concentrations of carbon and chromium at the measured melt temperature. 